A computed tomography (CT) scanner includes a rotating gantry rotatably mounted to a generally stationary gantry. The rotating gantry supports an X-ray tube and an array of detectors, which is mounted on the rotatable gantry opposite the X-ray tube, across an examination region. The rotating gantry and hence the X-ray tube and the detector array rotate around the examination region about a longitudinal or z-axis. The X-ray tube is configured to emit ionizing radiation that traverses the examination region (and a portion of a subject or object in the examination region) and irradiates the array of detectors. The array of detectors includes a plurality of detectors that detect the ionizing radiation and generate a signal indicative thereof. A reconstructor reconstructs an image, generating volumetric image data indicative of the portion of the subject or object in the examination region.
Wide band gap semiconductor detectors have been used to detect ionizing radiation for application such as security, non-destructive testing, and medical imaging. Unfortunately, such detectors are not well-suited for all imaging applications. By way of example, for Spectral CT, such detectors may have insufficient time resolution and response homogeneity and suffer from charge trapping and polarization. For most of the applications such detectors are required to be thick to provide the high stopping power necessary for absorption of high energy X-ray and γ-photons. The thickness of the detector crystal may have to exceed ten (10) millimeters (mm), typically three (3) to five (5) mm for CT and PET applications, with the lateral size ranging from ten by ten (10×10) to twenty by twenty (20×20) square millimeters (mm2). Obtaining a flawless detector crystal of that size with precisely controlled characteristics like composition, defect concentration, doping, etc. may not be readily attainable. Moreover, hundreds of volts are applied to the detectors to provide effective charge separation and collection.
The high voltage biasing and presence of inevitable crystal defects and imperfections lead to charge trapping inside the crystal and extended space charge regions formation, and, eventually, affect generation and collection of charge carriers produced by the ionizing radiation inside the detector crystal and the detectors response time, and decreases the signal-to-noise ratio and energy resolution of the detector. Another obstacle for the semiconductor detectors application in high flux radiation detection areas such as medical CT is that the X-ray flux dramatically changes during the scan which affects the detector crystal properties and can cause some undesirable effects such as signal pile-up, saturation, charge trapping, etc.
Techniques for reducing the charge trapping and polarization to improve the radiation detector performance, based on detector heating and sub-band-gap irradiation, are described in U.S. Pat. No. 5,248,885, U.S. Pat. No. 5,905,772, U.S. Pat. No. 7,312,458, U.S. Pat. No. 7,514,692, U.S. Pat. Nos. 7,652,258 and 7,800,071, and US Patent Application Publication 2010/0078559. However, both the heating and the illumination of the whole detector crystal greatly decreases electric resistance of the detector by generating additional charge carriers, which, in turn, increases the dark current and noise level, and requires considerable changes in the detector-coupled electronics. The non-homogeneous, but not patterned, IR illumination of the detector proposed in 2010/0078559, in addition to the above, also increases the detector response inhomogeneity.
Furthermore, the above techniques do not provide effective evacuation of holes, produced by ionizing radiation in detector regions situated deep inside the crystal and far from the cathode. The lack of an effective mechanism for a fast evacuation of holes from the crystal, irradiated with a high flux of ionizing radiation, leads to charge trapping and polarization and affects the detector response time making it insufficient for the high flux applications. Moreover, if the pixilated detector has on the anode side a steering electrode, which is intended for the faster evacuation of holes generated far from the cathode, illumination of such a detector increases electric conductivity of these sub-anode crystal regions and the pad-to-steering electrode leakage current by up to two orders of magnitude, which may hinder or even obstruct utilization of the steering electrodes proven to improve response time and energy resolution of the detectors.
In view of the foregoing, there is an unresolved need for other approaches to overcome deficiencies of semiconductor detectors in high flux imaging applications.